go-oncectx

This is a version of golang's sync.Once that allows calling Do with a context.

It is not expected that golang's stdlib will implement something like this, see golang/go#25312

Usage is like the sync.Once package, except that you must pass Do a context. The function called by the Do function is run in a separate goroutine and will always run to completion, regardless of whether the context's Done channel is closed. However, the Do function itself will return either when f returns or when the context Done channel is closed.

Why is this needed?

A naive approach would do something like so:

func (o *Object) initSomething(ctx context.Context) {
    o.once.Do(func() {
      CallSomethingWithContext(ctx)  
    })
}

However, a careful reading will see that if you have multiple goroutines which call initSomething, a cancellation of one of the caller's contexts will cause initialization to fail for all callers, which is probably not an intuitive result.